Internal combustion engines utilizing two or more different fuels have been proposed. As one example, the papers titled “Calculations of Knock Suppression in Highly Turbocharged Gasoline/Ethanol Engines Using Direct Ethanol Injection” and “Direct Injection Ethanol Boosted Gasoline Engine: Biofuel Leveraging for Cost Effective Reduction of Oil Dependence and CO2 Emissions” by Heywood et al. describe engines that are capable of using multiple fuels. Specifically, the Heywood et al. papers describe directly injecting ethanol into the engine cylinders to improve charge cooling effects, while relying on port injected gasoline to providing a majority of the combusted fuel over a drive cycle. The ethanol, in this example, can provide increased octane and increased charge cooling due to its higher heat of vaporization in comparison to gasoline, thereby reducing knock limits on boosting and/or compression ratio. This approach purports to improve fuel economy and increase utilization of renewable fuels.
The inventors of the present disclosure have recognized that requiring a user to re-fuel the engine system with two or more separate fuels (e.g., gasoline and ethanol), in order to achieve the advantages described by Heywood et al., can be burdensome. As one approach, United States printed publication number 2006/0191727 by Usami et al. describes an electric power generation system that includes a fuel storage unit having an ethanol permeable membrane for separating the ethanol from a mixed fuel including ethanol and gasoline. This publication describes how ethanol separation can be performed in proportion to the pressure difference across the permeable membrane and also according to the temperature difference across the membrane.
However, the inventors herein have recognized a variety of issues associated with the approach taken by Usami et al. As one example, the inventors have recognized that engine packaging constraints on-board a vehicle may preclude the addition of a fuel separator or may reduce the effective size of the separator. A reduction in the separator size can reduce fuel separation rates, which may in turn impair engine performance where the fuel to be separated is temporarily unavailable or available in a reduced amount due to the reduced separation rate. As another example, the use of a dedicated heater as taught by Usami et al. to improve the separation rate by way of heat addition is also limited by the similar packaging constraints.
To address these and other issues, the inventors herein have provided a fuel rail assembly for supplying pressurized fuel to a plurality of cylinders of an engine. As one example, the fuel rail assembly comprises a fuel rail housing defining an internal fuel rail volume having at least a first region and a second region; a fuel separation membrane element disposed within the fuel rail housing and segregating the first region from the second region, said membrane element configured to pass a first component of a fuel mixture through the membrane element from the first region to the second region at a higher rate than a second component of the fuel mixture; a fuel inlet disposed on the fuel rail housing, said fuel inlet configured to admit the fuel mixture to the first region; a plurality of fuel outlets disposed on the fuel rail housing, wherein each of said fuel outlets are configured to supply at least a portion of the fuel mixture from the first region to a respective one of said plurality of engine cylinders; and at least one membrane outlet disposed on the fuel rail housing and configured to supply at least a portion of the first component that has passed through the membrane element from the second region to a location external the fuel rail housing.
By placing the fuel separation membrane within the fuel rail assembly, which is in relative close proximity to the engine, the fuel separation membrane and the fuel mixture to be separated by the membrane can be at least partially heated by the engine. In this way, the fuel separation rate can be increased without requiring a separate heater, thereby reducing cost and other associated packaging constraints. Further, in this way, it is possible to utilize a fuel pump to pressurize fuel for injection to the engine, as well as for improved separation of the fuel mixture.
The inventors herein have also recognized that the fuel separation rate can be further increased by increasing the surface area of the fuel separation membrane relative to the volume of the separator. As one example, the surface area of the membrane may be increased for a given separator volume by supporting the membrane on a substrate that forms a non-planar membrane element within the fuel rail, such as a tube. By supplying the pressurized fuel to the external surface of the tubular membrane element, the substrate can be loaded in compression, which can provide an additional strength advantage for some substrate materials such as ceramics or other materials that are relatively stronger when loaded in compression than tension.
The inventors herein have also recognized that a plurality of separate membrane elements within a common fuel rail assembly can further increase the separation rate for a given separator volume. For example, by utilizing multiple smaller tubes for the membrane elements, the ring stress in the substrate can be reduced, thereby enabling a further reduction in wall thickness of the substrate. A reduction in wall thickness and increased surface area of the membrane elements can further increase the fuel separation rate while also reducing packaging constraints. These and other advantages will be appreciated in light of the following specification and accompanying drawings.
In still another embodiment, a method of operating a fuel system for an internal combustion engine may be used, comprising: supplying a pressurized fuel mixture to a fuel rail, said fuel mixture including a hydrocarbon component and an alcohol component; separating at least a portion of the alcohol component from the fuel mixture by passing at least said portion of the alcohol component through a fuel separation membrane element disposed within the fuel rail to obtain an alcohol reduced fuel mixture; delivering the alcohol reduced fuel mixture from the fuel rail to at least a plurality of cylinders of the engine via injectors fluidly coupled with the first fuel rail; and supplying the separated alcohol component to the engine. In this way, it is possible to utilize a fuel pump to pressurize fuel for injection to the engine, as well as for improved separation of the fuel mixture.